1. Field of the Invention
The present invention generally relates to a material composition used in detecting ionizing radiation, particularly gamma-ray and X-ray radiation, and methods for toughening crystals of these materials, as the crystals are grown. More particularly, the present invention relates to improved lanthanide halide compositions that introduce structural perturbations in the crystal lattice to toughen the crystal against formation of critical flaws along cleavage planes of the crystal.
Scintillators are materials that emit flashes or pulses of light when they interact with ionizing radiation. Scintillator crystals are widely used in detectors for gamma-rays, X-rays, cosmic rays, and particles characterized by an energy level of greater than about 1 keV. It is possible to make radiation detectors, therefore, by coupling the crystal with some means for detecting the light produced by the crystal when it interacts, or “scintillates,” when exposed to a source of radiation. The photo-detector produces an electrical signal proportional to the intensity of the light pulses received.
2. Related Art
A number of the scintillator materials in current use possess most of the important properties that define these materials as useful: properties such as high light output, high cross-section, short decay times, and minimum afterglow. Prior art examples used for many years include thallium-activated sodium iodide (NaI(T1)), bismuth germanate (BGO), cerium-doped gadolinium orthosilicate (GSO), and cerium-doped lutetium orthosilicate (LSO).
While each of these materials exhibit properties which are suitable for many applications, each also has its shortcomings. Problems such as mediocre light yield, poor physical strength, and the difficulty and expense of producing large, high quality crystals continue to plague the industry. Moreover, non-proportional light yield seriously limits spectroscopic performance of these materials.
A number of lanthanide-halide compounds have recently been described as promising scintillators to address the problem of light yield and proportional response. In particular:
U.S. Pat. Nos. 6,437,336 and 6,818,896 describe a monoclinic single crystal with a lutetium pyrosilicate structure. The crystal is formed by crystallization from a congruent molten composition of Lu2(1-x)M2xSi2O7 where Lu is lutetium or a lutetium-based alloy which also includes one or more of scandium, ytterbium, indium, lanthanum, and gadolinium; where M is cerium or cerium partially substituted with one or more of the elements of the lanthanide family excluding lutetium; and where x is defined by the limiting to level of Lu substitution with M in a monoclinic crystal of the lutetium pyrosilicate structure. The crystals are said to exhibit excellent and reproducible scintillation response to gamma radiation.
U.S. Pat. No. 7,060,982 describes a fluoride single crystal for detecting radiation having high luminescence intensity. The fluoride single crystal contains Ce and at least one element (R1) of Lu and Gd, wherein the single crystal is represented by the general formula Ce1-xR1xF3 where 0.001<x.<0.5.
U.S. Pat. No. 7,067,815 describes an inorganic scintillator material, a method for growing a single crystal of the scintillator material. The inorganic scintillator material has the general composition M1-xCexCl3, where M is selected among lanthanides or lanthanide mixtures, preferably among the elements or mixtures of elements of the group consisting of Y, La, Gd, Lu, in particular among the elements or mixtures of elements of the group consisting of La, Gd and Lu; and x is the molar rate of substitution of M with cerium, x being not less than 1 mol % and strictly less than 100 mol %.
U.S. Pat. No. 7,067,816 describes an inorganic scintillator material of general composition M1-xCexBr3, wherein: M is selected among lanthanides or lanthanide mixtures of the group consisting of La, Gd, Y in particular among lanthanides or lanthanide mixtures of the group consisting of La, Gd; and x is the molar rate of substitution of M with cerium, x being not less that 0.01 mol % and strictly less than 100 mol %.
U.S. Pat. No. 7,084,403 describes scintillator materials based on certain types of halide-lanthanide matrix materials are described. In one embodiment, the to matrix material contains a mixture of lanthanide halides, i.e., a solid solution of at least two of the halides, such as lanthanum chloride and lanthanum bromide. In another embodiment, the matrix material is based on lanthanum iodide alone, which must be substantially free of lanthanum oxyiodide. The scintillator materials, which can be in monocrystalline or polycrystalline form, also include an activator for the matrix material, e.g., cerium.
U.S. Pat. No. 7,129,494 describes fast scintillator materials capable of resolving the position of an annihilation event within a portion of a human body cross-section. In one embodiment, the scintillator material comprises LaBr3 doped with cerium. Particular attention is drawn to LaBr3 doped with a quantity of Ce that is chosen for improving the timing properties, in particular the rise time and resultant timing resolution of the scintillator, and locational capabilities of the scintillator.
The foregoing compounds, however, do not address the issue of crystal strength nor do they address the issue of producing large, fracture-resistant or crack-free single crystals. However, the lanthanide halides are generally soft, compliant crystals which would suggest that strength would not be an issue with these crystals. However, while soft, crystals of these materials are also highly anisotropic with a strong preference for slip along on hexagonal prismatic planes. We believe that slip on these prismatic planes results in dislocation pile-ups and critical-sized flaws that intensify the stress leading to cleavage fracture on these same planes. Lanthanide halide scintillators form hexagonal crystals in the uranium (III) chloride prototype structure, which is characterized by 9-fold coordination of to the cation, low c/a ratio, and open channels along the c axis. These materials while relatively “soft” tend to be very brittle, and fracture by cleavage along their prismatic planes, orthogonal to the c or basal plane. Analysis of this structure indicates dislocation formation energies and mobilities on these same prismatic planes are highly favored relative to the more usual basal plane slip; that is, the slip and cleavage planes are identical. This structure is therefore highly susceptible to cracking during crystal growth and subsequent detector fabrication.
Growth of lanthanide halide crystals is presently accomplished through various melt-solidification methods, wherein the crucible, or “boat,” of molten material is directionally cooled to nucleate and grow a crystal boule. Failure of the lattice due to critical flaws introduced by plastic strain is most likely during crystal growth or cool-down, due to the resultant longitudinal or radial thermal gradients within the crystal. Furthermore, the problem is exacerbated by anisotropy in the thermal expansion coefficient that is considerably larger in the [10•0] or a directions than in the [00•1] or c direction. Along the c-axis, thermal expansion occurs rather slowly at 7.5×10−6° C.−1, while expanding far faster along the a-axis at approximately 28.1×10−6° C.−1. This anisotropy in thermal expansion results in the formation of significant shear stresses on the prismatic [00•1] planes whenever a thermal gradient is applied more than 10° off the c-axis. We believe this anisotropy results in dislocation formation and motion and the formation of critical stress-concentrating flaws on these (weakest) planes of the crystal that subsequently lead to brittle fracture or cleavage. This, in combination with the propensity of these crystals to cleave along their slip planes, has been the chief impediment in achieving the necessary availability of large, cost effective to crystals.
Therefore, there remains an unmet need for providing a means for toughening lanthanide halides scintillating crystal compositions.